Cleaning pigs are propelled through a pipeline by a pressurized fluid as described, for example, in U.S. Pat. Nos. 3,389,417, 3,204,274 and 3,474,479. Some such pigs are made of a flexible, polyurethane foam, cylindrical body. It was believed essential that the body's outer cylindrical surface be only partially covered with spiraling bands of an abrasive material, such as a layer of steel bristles. The rear and front end walls of the cylindrical foam body are typically covered with an impervious coating designed to form a moving seal with the inner wall of the pipe. The pressure gradient between the pig's rear and front end walls, incurred by the pig due to its resistance to movement caused by the frictional drag on the wall of the pipe, causes the pig to shorten axially, whereby the portions of the foam body between the spiraling abrasive bands extend radially outwardly for the purpose of creating a moving seal between the foam body and the inner wall of the pipe. Both the rear end seal provided by the coating and the outwardly and radially extending foam portions between the spiraling abrasive bands are intended to prevent propelling fluid from flowing through or around the pig.
In practice it has been found that while the portions of the foam body between the spiraling abrasive bands do extend outwardly, the abrasive bands themselves become pushed inwardly by the inner wall of the pipe. The abrasive bands easily compress the foam thereunder and force it outwardly between the abrasive bands, thereby reducing the bands' abrasive effect. Since the foam is the weakest structural member of the pig's structure, the working life of the pig will be determined by the strength of the bare and exposed foam portions between the abrasive bands. But when the foam is stretched and contorted by the stresses produced thereon inside a pipeline, such as when the pig negotiates a bend or reduced opening, the foam easily tears because it poorly resists tensile forces.
Another problem with the conventional construction of such a pig is that it allows for no appreciable fluid flow through the bristles to clean them. The debris removed from the pipe's wall gradually collect between the bristles on the abrasive bands. Such debris further push the abrasive bands radially inwardly, thereby further reducing their abrasive effectiveness. Some of these debris become inhaled by the exposed open-cell foam and move inside the foam's pores. As a result, the collected solids greatly increase the original weight of the pig and reduce the flexibility of the foam. The debris that are not collected inside the bristles and that are not absorbed into the open cell foam are left behind the pig and remain in the pipeline, requiring at least another pig run.
In sum, the above enumerated conditions result in that the foam in conventional pigs becomes heavy, inflexible and torn, the bristles become clogged and ineffective, until the pig finally desintegrates and the abrasive bands tear away from the foam body.
In addition to the above, the relatively short working life of such known pigs is attributed to a variety of other causes, such as the irregularities of the inner wall of the pipe, including the pitted surfaces, circumferential and longitudinal weld seams, and the passage through bends, valves, restrictions, fittings, etc. As a result, the foam core is put in tensile stress causing it to wear, tear or become clogged with debris. Thereafter, the worn-out pig can only have a very limited working life.
In an attempt to prolong the working life of the known pigs, it was suggested to improve the seal between the foam and the inner wall of the pipe, by increasing the abrasive surface in contact with the wall of the pipe, and by leaving suitably-disposed openings in the abrasive surface through which the foam material can expand radially outwardly into wiping contact with the inner wall of the pipe. It was believed that these openings must be so constructed that the portions of the foam projecting outwardly therethrough should form an effective circumferential seal with the inner wall of the pipe, thereby preventing the flow of propelling fluid around the pig, between the rear and front end walls thereof, in the direction of propulsion.
Thus, the above described known pigs are all characterized by an attempt to maintain an effective seal between selected exposed portions of the foam and the inner wall of the pipe.
Unfortunately, the exposed foam portions in contact with the pipe's inner wall shorten the working life of the pig, and cause a relatively inefficient scraping action by the abrasive bands mounted on the outer cylindrical wall of the pig. These bands, in use, are pushed radially inwardly by the pipe and by the debris trapped between the bristles, while portions of the foam core between these bands move radially outwardly, thereby reducing the scraping effectiveness of the bristles. Other disadvantages of the prior art pigs of the above described character will subsequently become apparent.